Plugged in and booted up P3D (v.3.4.22.19868 - latest build)...no obvious difference in FPS between SLI and single card at default OR max settings, full-screen or windowed. I get slightly better FPS with a single card! (45-50 FPS SLI vs. 50-60 FPS single card)

SLI works fine. Confirmed w/ benchmarks.

Deleted and reinstalled latest drivers (381.65). No Joy. Rolled back to 378.78. No joy. Can't roll back to the popular 376.33, won't recognize 1080 TI.

Deleted and reinstalled P3D. No joy.

Assuming it's a driver/card issue and I'm out of luck but want to make sure I'm not missing something...? I should have more than enough CPU/RAM muscle. Thanks! :)

Without AA the difference is about 5%.
But with 4x SGSS enabled it helps holding the framerate instead of dropping way down. Especially with clouds .
Then 1 card compared to SLI dropps till almost 60% of the fps.

See Rob A's P3D guide at the P3D forum from Avsim for testing results.